In ex-pat areas in Spain rogue "gas maintenance" companies are quite common: they call and ask to check your gas systems. If you say the house is rented (as I did), they'll tell you the landlord asked them to come. *They* say things like "gas pipes need to be changed every x years", and they also have been known to make holes in pipes with a knife. Obviously they then charge you for the replacement work.

I just thought I'd mention that as I wouldn't be surprised if the suggestion that pipes need to be changed every 4 years originally came from one of these companies. I'm not a gas fitter though. You should be able to find a properly registered company in the yellow pages - make sure you use a company with a proper business address, even if it means paying more and do not pay money upfront whatever you do.